Method for making electric resistance heaters



METHOD FOR mum ELECTRIC RESISTANCE HEATERS Filed July 21, 1965 G. H.MOREY Sheet INVENTOR. GLEN H. MOREY FV///////////////A I METHOD FORMAKING ELECTRIC RESISTANCE HEATERS Filed July 21, 1965 G. H. MOREY Feb.4, 1969 Sheet INVENTOR.

GLEN MOREY United States Patent 3,425,864 METHOD FOR MAKING ELECTRICRESISTANCE HEATERS Glen H. Morey, Terre Haute, Ind., assignor toTempleton Coal Company, Terre Haute, Ind., a corporation of IndianaFiled July 21, 1965, Ser. No. 473,689 US. Cl. 117215 Int. Cl. H01b 3/124 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a methodfor making electric resistance heaters and is particularly concernedwith such a method and apparatus including the plasma spraying of theresistance heating element on a supporting surface, and also including,when desired, the spraying by plasma spraying of electrical insulatingmaterial employed in connection with the electric resistance heater.

The spraying of metals, ceramics, and the like by flame spraying devicesand the spraying of ceramics by plasma spraying is known but so far as Ihave been able to determine, the plasma spraying of resistance heatingelements has never heretofore been practiced by specific method claimed.

Plasma spraying, in brief, consists of creating a jet of plasma which isdirected toward a supporting surface while introducing into the jet thematerial to be deposited on the surface in finely granulated form. Theplasma is created by passing gas through a passage and within whichpassage the gas is ionized and raised to an elevated temperature bypassing an electric current therethrough. Such gases are quite hot, onthe order of up to 50,000" F. and issue from the passage at atemperature within the aforementioned range and at relatively highvelocity. =When particulate material is introduced into such a jet, thematerial will be softened by the heat of the jet and will be conveyed bythe jet to the surface on which the jet impinges and be deposited on thesurface in the form of a layer, the thickness of which can readily becontrolled.

With the foregoing in mind, a primary object of the present invention isthe provision of a method for employing plasma spraying to effectdeposition of electric resistance heating elements on a substrate whichmay take substantially any form, including metal and electricalinsulating materials.

Another object of this invention is the provision of a method for makingelectric resistance heating elements while simultaneously disposing theelectric resistance heating elements in the region where they are to beemployed.

Still another object of the present invention is the provision of amethod for forming electric resistance heating elements which will bemore resistant to deterioration than resistance heating elements madeaccording to the prior art methods.

Still another object of the present invention is the provision of amethod for making an electric resistance heat- 3,425,864 Patented Feb.4, 1969 ing element while placing the electric resistance heatingelement in intimate thermal contact with the member it is to heat.

The foregoing objects as well as still other objects and advantages ofthe present invention will become more ap parent upon reference to thefollowing specification taken in connection with the accompanyingdrawings, in which;

FIGURE 1 is a somewhat diagrammatic perspective view showing an electricresistance heating element being made according to the presentinvention;

FIGURE 2 is a somewhat schematic longitudinal section through a plasmagun and is [generally indicated by line II-II on FIGURE 1;

FIGURE 3 is a transverse cross-section indicated by line III-III onFIGURE 2;

FIGURE 4 is a sectional view through a support member showing resistanceheating element formed thereon according to the present invention;

FIGURE 5 is a perspective view showing a tubular electric heatingelement constructed according to the present invention;

FIGURE 6 is a fragmentary view showing the appearance of an electricresistance heating element constructed according to the presentinvention directly on the wall of a structure which is to be heated bythe heating element;

FIGURE 7 is a sectional view indicated by line VII- VII on FIGURE 6;

FIGURE 8 is a fragmentary view, partly in section, showing another typeof tubular heating element constructed according to the presentinvention;

FIGURE 9 is a sectional view indicated by line IX]X on FIGURE 8; and

FIGURE 10 is a fragmentary sectional view showing a resistance heatingelement consisting of a plurality of layers.

Referring to the drawings somewhat more in detail, in FIG. 1, 10indicates a plasma gun. Connected to the gun is a conduit 12 for thesupply of gas thereto, the said supply coming from a pressurized source14 and being under the control of a valve 16. The gas may comprisesubstantially any gas, but is preferably in the form of a substantiallychemically inert gas so that when the plasma jet issues from the gun,the plasma, in addition to the heating and conveying effect thereof,will also serve as a shielding medium for material therein.

Within gun 10 the gas is converted to plasma b electrical energysupplied to the gun via electric coupling 18.

Associated with the gun are the sources 20 and 22 of finely granulatedor particulate material which are connected to the gun near thedischarge end thereof by conduits 24 and 26 in which are disposed flowcontrol elements such as the valves 28 and 30.

The plasma jet is schematically indicated at 32 and it comprises arelatively high velocity, high temperature jet of plasma in which theparticulate material is entrained. This jet is directed toward andimpinges upon a support member 34. The support member 34 maybe metal orelectrical insulating material or any other sort of receiver which willwithstand the temperature of the plasma jet. The entire arrangement ofthe support member and at least the discharge end of the plasma gun maybe contained within an enclosure 36 which may have the atmospherethereof controlled as by the conduits 38 and 40 connected thereto.Generally, the enclosure is evacuated and back filled to a pressure ofabout 1 atmosphere with an inert gas.

Referring to FIGURE 2, it will be seen that the gun 10 comprises a body42 and extending through the body is a passageway 44 for gas. Electrodes46 and 48 are provided which can be energized with electrical energy todevelop a spark therebetween to initiate ionization of the gas flowingthrough the gun.

Following initial ionization, electrons accelerated from the backelectrode (cathode) to the front electrode (anode) as a result of thecurrent flowing therebetween cause further ionization of the gas. Thecontinued maintenance of electric current insures that plasma and theintense heat resulting therefrom will issue from the discharge end ofthe gun defined by the front electrode.

The electric current will maintain the gas ionized and at hightemperature, so that the gas discharged at the discharge end 50 of thegun is in the form of intensely hot plasma. Cooling water for theelectrodes is conveyed through the gun and conduit means 47 and passagemeans The temperature of the plasma is sufiicient to soften even ceramicmaterials and will furthermore soften and sometimes even substantiallyliquefy metallic materials introduced by way of conduits 24 and 26 intothe gas stream. These materials, when introduced into the gas stream areconveyed thereby to the support member and will be deposited thereon. Ifthe gun is moved, the materials will appear as a layer on the supportmember of a thickness depending upon the speed of movement of the gun orupon the number of times the gun is caused to traverse a given area ofthe support member.

The materials may be supplied simultaneously from both of sources 20 and22 or may be supplied individually from the respective sources, or insuccession. For example, one of the sources could comprise insulatingmaterial such as ceramic and this material could be deposited on asupport member first, to form an insulating base to receive theconductive resistance material. This procedure would be followed wherethe support member was itself conductive as, for example, where thesupport member was a metal. On other occasions, the conductive electricresistance heating element could be supplied first, as where the supportmember was in the form of electrical insulating material, alumina, forexample. In this case, the insulating material from the other sourcemight thereafter be supplied to the gun to form an insulating sealingcoating over the layer of electric resistance heating material.

Still further, the application could be, first, a layer of insulatingmaterial, then a layer of resistance heating material and, finallyanother layer of insulating material. It is understood that two or moreconstituents could be supplied to form a mixed layer.

Terminals could be provided on the support member which the electricalinsulating material contacts when applied to the support member, orregions of the resistance heating material could be left exposed andelectrical connection made thereto after the heater was manufactured.

In FIGURE 4 there is shown a support member 52 having terminals 54therein and electric resistance heating material 56 applied to thesupport member by plasma spraying and so applied as to make electricalconnection with the terminals 54.

FIGURE 5 shows how a tubular resistance heating element could befabricated according to the present invention. In FIGURE 5, 60 is atubular member of electrical insulating material, alumina, for example,which is also resistant to heat. Sprayed on member 60 is a layer 62 ofelectric resistance heating material. At spaced points along material62, preferably near the ends thereof, straps 64 are employed to whichelectric current supply leads 66 are connected so that an electriccurrent can be passed through th resistance heating material 62.

The arrangement of FIGURE 5 has proved to be a particularly advantageousmanner of constructing electric resistance heating elements frommaterials such as molybdenum disilicide. This material has always beendiflicult to fabricate into a resistance heating element. It is aconductive ceramic material and has heretofore been made into resistanceheating elements by a sintering process. This process forms resistanceheating elements which perform satisfactorily up to a certain limitingtemperature, but beyond this latter temperature the sintered re- 7sistance heating elements of the said material become weak and willcommence to deform, while, furthermore, some of the bonding agentemployed during the sintering process will boil off and the heatingelements will commence to become porous and lose strength andeffectiveness for this reason. Heating elements of this materialconstructed as shown in FIG. 5 have the merit that they are supported bythe substrate on which they are built up and can thus be operated athigher temperatures and for longer periods of time than the sinteredelements according to the prior art, as referred to above.

FIG. 6 shows the wall 70 of a furnace or other enclosure which is to beheated electrically, and directly on this wall is formed an electricresistance heating element 72 by the practice of the present invention.This resistance heating element may be applied by causing the plasmaspray gun to traverse the pattern illustrated, but, alternatively, thewall 70 can be masked off and the resistance heating material will thenbe applied to the exposed portion only of wall 70. Connection to spacedpoints of the heating element illustrated can be effected as by bolts 74to which electric leads 76 are connected. A detail of such a connectionis illustrated in FIG. 7.

FIGS. 8 and 9 show how a metal pipe 80 could be provided with anelectric resistance heating element constructed directly thereon. Inthese figures, the metal pipe 80 has applied thereto a first layer ofelectrical insulating material at 82. This layer which is applied byplasma spraying, may comprise, for example, a ceramic.

A second layer 84 is applied by plasma spraying on top of layer 82 andthis last-mentioned layer is electrically conductive material. The layer84 is preferably stepped inwardly from the end of layer '82.

A third layer 86 of electrical insulating material may then be plasmasprayed on top of conductive layer 84 and is also preferably steppedinwardly from the end of layer 84. The exposed end regions of layer 84receive straps 88 by means of which electrical connection can be made tothe conductive layer. Alternatively, layer 86 can be extended to beyondthe end of layer 84 and can also be applied over strap 88 except for theterminal portions thereof. Dotted line 90 shows layer 86 applied in thislast-mentioned manner.

In FIG. 10 a metal pipe 90 is illustrated and directly on the surface ofmetal pipe 90 is a layer 92 of ceramic. Directly applied over layer 92is a layer 94 which is a mixture of ceramic and metal, known in thetrade as Cermet and which is a material consisting of ceramic and metalso admixed that the metal is in continuous phase in the ceramic andthereby can con-duct electricity and thereby form an electric resistanceheating element.

The final layer illustrated in FIG. 10 at 96 is again ceramic similar tolayer 92, and, therefore, non-conductive. The arrangement shown in FIG.10 could be arrived at by spraying, by the plasma jet, the ceramiccontinuously on pipe 90 and during a portion of the time that theceramic is being sprayed on the pipe, metal particles are introducedinto the plasma jet in sufiicient quantity to form the conductive layer94. This can readily be accomplished because the plasma jet can becontinued while the supply of particulate material thereto is regulated,including initiating d interrupting the supply of one or more materialsto t e plasma jet, or while modifying the proportions of the materialsapplied relative to each other.

The present invention provides, as will be seen from the foregoing, arelatively simple method and apparatus for making electric resistanceheaters characterized in the advantages that electric heaters ofimproved quality can be made, and that heaters can be formed in situwhere they are to be used, and resistance heaters that at the presenttime cannot be fabricated, or can only be fabricated with greatdifficulty on account of the temperatures required, can easily be madeby the method of the present lnvention.

Other advantages of the present invention are:

(1) It makes possible the use of many materials which are electricallysuitable for resistance heating elements which otherwise could not beused because of the mechanical difiiculty of fabricating the materialinto rods and other shapes.

(2) The heating element applied according to the present invention isspread over the entire surface so that heat emission is uniform.

(3). Because the heating element is spread over a large surface it canoperate at a much lower temperature than if it were in the form of rodswhile at the same time maintaining an adequate temperature of the openspace in a furnace or oven, or in a body being heated.

(4) In conventional electrical furnaces the heat must usually betransferred from the heating element through a refractory wall. With thenew method the heating element can face into the area to :be heated sothat the heat does not have to pass through a refractory wall, thusresulting in a lower temperature for the heating element, longer lifethereof, and more effective operation, as well as greater reservecapacity.

It will be understood that this invention is susceptible to modificationin order to adapt it to different usages and conditions; andaccordingly, it is desired to comprehend such modifications within thisinvention as may fall within the scope of the appended claims.

What is claimed is:

1. A method of forming electric resistance coatings on a support havingan alumna surface by introducing a chemically inert gas into a closedend of a tubular member of ceramic electric insulation material,conducting a current of electrons into the gas in the tubular member bymeans of a cathode while conducting electrons away from said gas bymeans of an anode at a point downstream from the cathode and at a ratesufficient to ionize the gas and convert it into a hot plasma,introducing finely granulated molybdenum disilicide into the plasma at apoint downstream of the anode, and directing the current of hot plasmaand molybdenum disilicide upon a surface of alumina in an atmosphere ofinert gas.

2. The method of claim 1, in which the alumina surface is on the outsideof a metal tubular member.

3. The method of claim 1, in which electric terminals are mounted uponthe alumina surface before the molybdenum disilicide is applied.

4. The method of claim 3, in which a final coating of alumina is appliedupon the molybdenum disilicide coating by introducing powdered aluminainto the plasma immediately after the molybdenum disilicide has beenapplied.

References Cited UNITED STATES PATENTS 3,109,228 11/1963 Dyke et al117212 XR 3,183,337 5/1965 Winzeler 117-93.1 3,197,335 7/1965 Leszynski117212 3,269,856 8/1966 Jones 11793.l XR 3,208,835 9/1965 Duncan et a1.117-217 XR 3,247,579 4/1966 Cattermole et a1. 11793.1 3,279,939 10/1966Yenni l1793.1 3,347,698 10/1967 Ingham 11793.1

WILLIAM L. JARVIS, Primary Examiner.

US. Cl. X.R.

